This invention relates to a modem signal transmission and reception apparatus which on one hand converts an analog signal such as voice or a carrier of a modem into digital data and transmits the digital data together with dial information and other information and on the other hand receives reception data from an opposing apparatus, and more particularly to a transmission apparatus, a reception apparatus and a communication apparatus as well as a modem signal transmission and reception method suitable for use with communication between apparatus connected to each other by a toll network connected via a metal circuit or an in-band ringer.
Generally, where communication is performed by a dedicated line such as a metal line between a telephone set or a FAX terminal accommodated in a private branch exchange (PBX) and another telephone set or another FAX terminal accommodated in another private branch exchange, an in-band ringer (IBR) or a modem (modulator-demodulator) is interposed on the dedicated line which interconnects the private branch exchanges as seen in FIG. 31.
Referring to FIG. 31, a private branch exchange 103 is connected to a dedicated line 111 as a toll network by an in-band ringer 104 and a modem 105 while another private branch exchange 108 is connected to the dedicated line 111 by another in-band ringer 107 and another modem 106.
The in-band ringer 104 performs signal conversion processing of dial information and other necessary information between the private branch exchange 103 and the dedicated line 111. Similarly, the in-band ringer 107 performs signal conversion processing of dial information and other necessary information between the private branch exchange 108 and the dedicated line 111. Both of the in-band ringers 104 and 107 use a single tone signal of, for example, approximately 3 kHz as a dial signal to be outputted to the dedicated line 111 side.
The modem 105 is connected to the opposing modem 106 via the dedicated line 111, and, on one hand, transmits a transmission analog signal from a terminal 101 or 102 on the private branch exchange 103 to the modem 106 and, on the other hand, receives an analog signal from a terminal 109 or 110 on the opposing private branch exchange 108 side.
Similarly, the modem 106, on one hand, transmits a transmission analog signal from the terminal 109 or 110 on the private branch exchange 108 side to the modem 105 and, on the other hand, receives an analog signal from the terminal 101 or 102 on the opposing private branch exchange 103 side.
Consequently, communication can be performed between a terminal accommodated in the private branch exchange 103 (for example, a FAX terminal 101 or a telephone terminal 102) and another terminal accommodated in the other private branch exchange 108 (for example, a FAX terminal 109 or a telephone terminal 110) via the in-band ringers 104 and 107, the modems 105 and 106 and the dedicated line 111.
Each of the modems 105 and 106 particularly includes such a modem signal transmission section 112 and a modem signal reception section 119 as shown in FIGS. 32 and 33, respectively.
Referring to FIG. 32, the modem signal transmission section 112 in the modem 105 includes an A/D conversion section 113, a demodulation section 114, a carrier generation section 115, a roll-off filter 116, a discrimination section 117 and a modem transmission processing section 118 (while the following description is given of the construction of the modem signal transmission section 112 paying attention to the modem 105, also the modem 106 has a similar construction).
The A/D conversion section 113 receives an analog signal (for example, of the voice band of 0.3 to 3.4 kHz) inputted from a FAX terminal 101 or a telephone terminal 102 via the private branch exchange 103 and the in-band ringer 104 and converts the analog signal into a digital signal at a sample rate of, for example, approximately 12 kHz.
The demodulation section 114 provides, to digital data from the A/D conversion section 113, angle information having a carrier frequency of, for example, approximately 1,700 Hz generated by the carrier generation section 115 for each predetermined angle to demodulate the analog signal into a base band signal.
The roll-off filter 116 performs decimation processing for a demodulation signal from the demodulation section 114 to effect conversion from a sample rate (approximately 12 kHz) into a symbol rate (for example, approximately 3,000 Hz) to decrease the amount of information and has a function as a filter for decimation processing.
It is to be noted that the frequency band of 1,700 Hz generated by the carrier generation section 115 and the frequency band of 3,000 Hz after conversion by the roll-off filter 116 described above can be used, taking the voice and the frequency bands prescribed in the Recommendations V.29, V.22 bis, V32 and so forth into consideration, commonly with them. Particularly, the demodulation frequency band of 1,700 Hz corresponds to the carrier frequency of the G3 FAX in the Recommendations V.29.
The discrimination section 117 receives a signal to which decimation processing has been performed by the roll-off filter 116, and discriminates coordinates on a two-dimensional plane to convert the amount of the signal having been demodulated into a base band signal into an amount of information necessary to transmit the signal on the dedicated line 111.
The modem transmission processing section 118 performs, for digital data as a result of the discrimination from the discrimination section 117, modem processing such as, for example, gray/natural conversion, finite summing, signal point generation, roll-off filter processing, modulation processing and D/A conversion processing. The modem signal (analog signal) obtained by such modem processing is transmitted to the modem 106 via the dedicated line 111.
Referring now to FIG. 33, the modem signal reception section 119 of the modem 106 includes a modem reception processing section 120, a roll-off filter 121, a modulation section 122, a carrier generation section 123, and a D/A conversion section 124 (while the following description is given of the construction of the modem signal reception section 119 paying attention to the modem 106, also the modem 105 has a similar construction).
The modem reception processing section 120 performs, for a signal received via the dedicated line 111 from the modem transmission processing section 118 of the opposing modem 105, reverse modem processing such as A/D conversion processing, demodulation processing, roll-off filter processing, automatic gain control, automatic equalization, carrier phase correction, signal point discrimination, and finite summing.
The roll-off filter 121 has a function as a filter for interpolation processing which performs interpolation processing (processing reverse to the decimation processing of the roll-off filter 116 on the transmission side described above) for a signal to which the reverse modem processing has been performed from the modem reception processing section 120.
By the filter processing by the roll-off filter 121, the original amount of information can be restored through conversion from a symbol rate (for example, approximately 3,000 Hz) to a sample rate (for example, approximately 12 kHz).
The modulation section 122 modulates an output signal of the roll-off filter 121 with a carrier frequency signal of approximately 1,700 Hz generated by the carrier generation section 123 to convert the base band signal back into a signal of the original frequency band (for example, for a voice signal, 0.3 to 3.4 kHz).
The D/A conversion section 124 converts a digital signal modulated by the modulation section 122 into an analog signal. The analog signal obtained by the conversion is outputted as an output of the modem signal reception section 119 to the in-band ringer 107.
In the communication system having the construction described above with reference to FIG. 31. When communication is to be performed, for example, between telephone terminals 102 and 110 or between FAX terminals 101 and 109, dial information for which signal conversion has been performed by the in-band ringers 104 and 107 is communicated between the private branch exchanges 103 and 108, and an analog signal such as a voice signal or a FAX signal is communicated via the modems 105 and 106 and the dedicated line 111.
For example, a voice signal as an analog signal from a telephone terminal 102 (or a FAX signal from a FAX terminal 101) is converted into a digital signal by the A/D conversion section 113 of the modem 105, demodulated by the demodulation section 114 and discriminated in regard to coordinates by the discrimination section 117.
Digital data as a result of the discrimination are transmitted, after they undergo modem processing by the modem transmission processing section 118, to a telephone terminal 110 of the other party via the dedicated line 111, modem 106, in-band ringer 107 and private branch exchange 108.
Further, for example, the discrimination section 117 of the modem signal transmission section 112 in the modem 105 performs discrimination not only of such an analog signal as described above but also of a tone signal (for example, of approximately 3 kHz) from the in-band ringer 104, and transmits the tone signal to the in-band ringer 107 via the dedicated line 111. The in-band ringer 107 recognizes the tone signal from the in-band ringer 104 of the transmission side.
However, since the modems 105 and 106 in such a communication system as shown in FIG. 31 are limited in number of discrimination points which form a discrimination plane in the discrimination section 117, when a tone signal is to be transmitted, if the output of the roll-off filter 116 is used as it is for discrimination, then a discrimination error which does not satisfy an S/N ratio (signal to noise ratio) required for the tone signal sometimes occurs.
In other words, if the roll-off filter output regarding a tone signal from the in-band ringer 104 (107) on the origination side is used as it is for discrimination by the discrimination section 117, then discrimination sometimes results in failure because of a discrimination error which satisfies the required S/N ratio.
The communication system has a subject to be solved in that, in this instance, the in-band ringer 107 (104) on the termination side cannot recognize the tone signal from the origination side and the dial information from the transmission side cannot be notified accurately to the reception side.